1. Field of the Invention
The present invention relates generally to a MOS composite type semiconductor device driven by a MOS transistor and more particularly to a power semiconductor device such as an insulated gate bipolar transistor (IGBT), a MOS controlled thyristor (MCT), a MOS advanced gate turn-off thyristor (MAGT), or a power MOSFET.
2. Description of the Related Art
AMOS composite type semiconductor device driven by a MOS transistor is used as a power semiconductor device, in order to gain a high drive power with a low control voltage. In addition, in order to gain a large current, a structure in which a plurality of segments are arranged to form a device is adopted. FIG. 1 shows a pressure-contact type IGBT as an example of this type of power semiconductor device. In a pellet substrate 110, a middle-sized or small-sized IGBT element is used as one segment, and a plurality of segments 120 are arranged to constitute an IGBT. This type of IGBT is disclosed in, for example, Jap. Pat. Appln. KOKAI Publication No. 3-218643 (corresponding to U.S. Pat. No. 5,376,815).
As is shown in FIG. 2, each segment 120 is provided with a polysilicon gate electrode layer 122 having a plurality of openings 121. The polysilicon gate electrode layer 122 provided independently for each segment 120 is extended to a central portion of the pellet substrate 110 and is electrically connected to a gate electrode lead-out portion 123 formed of, e.g. A1 at the central portion of the substrate 110.
With reference to FIG. 3, the cross-sectional structure of the IGBT and a method for manufacturing the IGBT will now be described. FIG. 3 is a cross-sectional view taken along line 3--3 in FIG. 2 showing the segment 120. FIG. 3 shows the state in which the pellet substrate is interposed between an emitter pressure-contact plate put in pressure contact with an emitter electrode layer and a collector pressure-contact plate put in pressure contact with a collector electrode layer.
At first, impurity ions are injected in a bottom surface portion of an N.sup.- semiconductor substrate 131, thus forming a bottom-side P emitter layer 132, and impurity ions are selectively injected in a major surface portion of the N.sup.- semiconductor substrate, thus forming a P.sup.+ region 133. Then, the surface of the substrate 131 is thermal-oxidized and an oxide film is formed. After a polysilicon layer is formed on the oxide film, a gate oxide film 134 and a polysilicon gate electrode layer 122 are formed by patterning. In this case, the polysilicon gate electrode layer 122 is divided into the respective segments 120 shown in FIG. 2. Each segment has a pattern with openings 121. Impurity ions are injected in the substrate 131 through the openings 121 and P base regions 135 are formed. After a mask (not shown) is formed on bottom portions of the openings 121, impurity ions are injected in the P base region 135. Thus, N emitter regions 136 are formed. An oxide film 137 is formed on the resultant structure and regions of the oxide film 137 among the gate electrode layers 122 are selectively removed. Thus, portions of the N emitter region 136 and the P base regions 135 (P.sup.+ region 133) among the N emitter regions 136 are exposed. An emitter electrode layer 138 is formed on the oxide film 137 and is electrically connected to the N emitter region 136 and P.sup.+ region 133. A gate electrode lead-out portion 123 is formed and is electrically connected to the polysilicon gate electrode layer 122. Furthermore, a collector electrode layer 139 is formed on the bottom-side emitter layer 132.
Subsequently, the characteristics of the IGBT are checked in units of segment 120. If there is a segment with defective characteristics, the segment is repaired so that it will not function (i.e. the segment with defective characteristics is separated from normal segments). Then, an annular emitter pressure-contact plate 141 is provided on the emitter electrode layer 138, and a collector pressure-contact plate 142 is provided on the collector electrode layer 139. The pellet substrate 110 is interposed between both pressure-contact plates 141 and 142 under pressure acting in directions P1 and P2.
A method of repairing the defective segment will now be described in detail with reference to FIG. 4. If there is a defective segment 120a, as indicated by X-mark 150, a narrow portion (indicated by .smallcircle.-mark 160) of the polysilicon gate electrode layer near the connection part of the segment 120a with the gate electrode lead-out portion 123 is cut off by means of dry etching, etc. Thus, the defective segment 120a is separated from the gate electrode lead-out portion 123 and is rendered non-conductive.
However, in order to cut off the defective segment by means of dry etching, it is necessary to perform etching by forming a mask pattern having a hole corresponding to the cut-off portion 160 of the defective segment 120a. Thus, a process of repair is time-consuming.
In the structure as shown in FIGS. 1 to 3, when the defective segment 120a is repaired, the entire segment 120a does not function although the defective portion (x-mark 150) is only a part of the segment 120a (for example, one cell). Since the area which one segment occupies in the pellet is relatively large, the current drive performance after the repair deteriorates greatly in the above repairing method. If there are two or more defective segments, the current drive performance further deteriorates. Thus, in order to gain a sufficiently high current drive performance, it is necessary to enlarge the area of the pellet in consideration of a decrease in current drive performance due to the defective segments.